1. Field of the Invention
This invention relates to the epitaxial growth of graphene, and more particularly to the use of a carbon ion beam to grow isotopically-enriched graphene and isotope-junctions.
2. Description of the Related Art
Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb, hexagonal crystal lattice. Graphene is a basic building block for graphitic materials of all other dimensionalities. Graphene can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite.
Graphene has rather remarkable properties. Graphene is stable, chemically inert, and crystalline under ambient conditions. It is a semimetal in that its conduction and valence bands just meet at discrete points in the Brillouin zone. An electron in graphene has an effective mass of zero and behaves more like a photon than a conventional massive particle. Finally graphene can carry huge current densities—about 108 A/cm2, roughly two orders of magnitude greater than copper. Graphene is a candidate for replacing silicon as a basis for faster, more powerful electronics. Graphene nanoribbons (GNRs) are essentially single layers of graphene that are cut in a particular pattern to give it certain electrical properties.
The 2010 Nobel Prize in Physics was recently awarded to Andre Geim and Konstantin Novoselov for their research into graphene. Geim and Novoselov discovered that by applying Scotch tape to graphite they could pull off thin flakes that consist of one, several or many layers of graphene. To locate the rare one-layer flakes, they took advantage of an optical effect: if the flakes are deposited on silicon dioxide substrate of just the right thickness, one-layered graphene reveals itself through interference fringes. It's too early to say whether graphene could end up being useful. Exploiting its unusual electronic properties could prove too difficult to pull off in a cost-effective way.
Epitaxy refers to the method of depositing a monocrystalline film on a monocrystalline substrate. The deposited film is denoted as an epitaxial film or epitaxial layer. The term epitaxy comes from the Greek roots epi, meaning “above”, and taxis, meaning “in ordered manner”. It can be translated “to arrange upon”. Epitaxial films may be grown from solid, gaseous or liquid precursors. Because the substrate acts as a seed crystal, the deposited film takes on a lattice structure and orientation identical to those of the substrate. Techniques for epitaxy deposition include but are not limited to Molecular Beam Epitaxy (MBE), Chemical Vapor Deposition (CVD) and plasma assisted CVD.
These epitaxy techniques have been used to a create single or a few sheets of graphene. A substrate is provided that includes a single crystal region on the surface of the substrate. This region has a honeycomb, hexagonal crystal lattice substantially lattice-matched to graphene. Carbon atoms are deposited at the vertices of three adjacent atoms in the substrate. The distance between these vertices is approximately the bond length of graphene.
Graphene sheets have been epitaxially grown using both natural and enriched-carbon precursors. Natural carbon comprises a mixture of approximately 99% 12C, 1% and 13C and trace amounts of 14C. Natural carbon may be enriched to, for example, include any desired amount of 13C, up to 99.9% given enough time and money. Epitaxially-grown graphene sheets using conventional methods mirror the isotopic composition of the precursor and may often include other non-carbon contaminants from the precursor in the graphene lattice.